The present invention relates to thin polymer films with a concavity which serves as a receptacle to position a sample for chemical analysis. The concavity is drawn in the film by placing an open tube in contact with the film at the desired position and creating a mild vacuum in the tube to stretch the polymer film slightly beyond its elastic limit, or by other suitable means. The resulting concavity remains after the removal of the tube or other forming device.
Various methods of spectral analysis of chemicals (e.g. x-ray fluorescence, XRF) use a cup as a container of liquid samples. Torrisi, in U.S. Pat. No. 4,587,666 (1986), describes a method of making a sample cup with thick walls and a membrane to separate the liquid sample from the vacuum chamber of the XRF instrument. The membrane is large (1 to 2 inches) and must have a "perfectly flat horizontal face." The container can support a liquid or solid and separate it from the vacuum portion of the instrument while passing at least some of the illuminating and emitted radiation.
An alternative method of supporting a sample with a low vapor pressure is to attach it to a thin, flat membrane and support the entire assembly in the vacuum chamber of the instrument. The probe beam can be electromagnetic radiation (e.g. x rays) or particles (e.g. accelerated protons). For small samples, micro-beams (diameters of less than 2 mm) have been used. This puts the support membrane under intense radiation which could lead to damage and failure.
The characteristics of the membrane are important for the analysis in the following ways, a partial list from Solazzi in his article entitled "Xray Fluorescence Thin-Film Sample Support Materials," American Laboratory, (1985), 17(11):3):
1. Relatively high degree of resistance to chemical attack.
2. Resistance to radiation damage such as embrittlement, thermal softening, etc. PA1 3. Good sample retention strength so the dried sample does not fall off or blow off. PA1 4. Freedom from interfering impurities. PA1 5. Thin, and yet strong enough to withstand handling. PA1 1. The position of the small, dry sample is uncertain (e.g. soaking on filter paper); PA1 2. The sample falls off during handling; PA1 3. The holder deteriorates because of radiation damage or high temperature or both; and PA1 4. Upon drying, the holder allows the sample to clump in such a way that the probe beam and radiation emitted by the analyte do not correctly represent the quantity of sample.
Support films typically limit the detection of small quantities of analyte because the films scatter photons or particles from the probe beam. In XRF, scattering is the limiting factor. In response, over the past 40 years workers have attempted (with limited success) to make thinner films of low atomic weight materials (carbon, nitrogen, oxygen, hydrogen and boron). Examples are boron nitride as proposed by Prang, et al., ("Boron Nitride Sample Carriers for Total-Reflection Xray Fluorescence," Spectrochemica Act, (1993), 48B: 153-161), and by Pauwels, et al., ("Polyimide Substrate for Nuclear Targets," Nuclear Instruments and Methods, (1979), 167: 109-112). The thicker the film, the more the film becomes a limiting factor in the analysis. Those skilled in the art will recognize that techniques for preparing thin polymer films (i.e. films of less than about 50 .mu.g/cm.sup.2) are well known.
Many samples are solutions or suspensions. The solvent is typically evaporated (see Hannson, et al., "A Non-Selective Preconcentration Technique for Water Analysis by PIXE," Nuclear Instruments and Methods in Physics Research, (1984), B3: 158-162 and Mangelson, et al., "Particle Induced X-ray Emission Elemental Analysis: Sample Preparation for a Versatile Instrumental Method," Scanning Microscopy, (1990), 4: 63-72). Flat sample holders are often made of a porous material (e.g. filter paper). Even though the sample droplet may be carefully placed, there remains ambiguity in the position of the sample after drying because of uncontrolled amounts of lateral diffusion and soaking of the solution which has been applied. Even when the film is smooth and impervious to liquid, the small sample may not dry exactly in its initial position because of droplet motion. Slight inhomogeneities in the surface tension of the film may induce the droplet wet and spread in an uncontrollable manner. Additionally, evaporation may deposit the solid at the edge of the evaporating droplet. These processes take an unknown portion of the sample out of the microscopic probe beam, and invalidate the analysis.
Over the past 40 years, a long-felt need has become evident because all of the conventional approaches suffer from one or more of the following problems:
This last problem arises because a large clump can severely attenuate the probe radiation (leaving some of the sample unexcited), or it can attenuate the emitted radiation. In addition, emission of one element in the sample can be absorbed by another and enhance the emission of the second, thus invalidating the analytical method. It is very difficult to correct for these effects from sample to sample because the clump size (and therefore the error) may vary from place to place because of random amounts and positions of clumping. A thin film of the analyte minimizes these errors and allows for a dependable calibration of their extent. The severity of this self-absorption problem scales as the absorption coefficient, so the problem can be largely ignored for highly transparent samples.
It would be advantageous if there were precisely located positions on the film where the sample droplet would remain during evaporation. The films should be free of any interfering contaminants. In addition, the film should hold the dry sample in position during handling and analysis. These and other advantages are achieved with the instant invention which is described in more detail to follow.